The present invention relates to the fabrication of metamaterials.
Metamaterials are periodically repeating, synthetic composite structures that are specifically engineered to circumvent inconvenient bulk material properties. Photonic bandgap crystals such as synthetic opals are a subset example of metamaterials. The exceptional characteristics and response functions of metamaterials are not observed in the individual constituent materials of the composite, and these phenomena arise as a direct result of the periodic inclusion of functional materials such as metals, semiconductors or polymers embedded within the composite. However, the fabrication of such structures is a serious experimental challenge as this full three-dimensional deposition and patterning requirement is extremely difficult to satisfy using conventional techniques such as chemical vapour deposition and photolithography.
Holey optical fibres exploit the concept of using a periodic array of air holes to define the transverse refractive index profile of the fibre. These fibres have exhibited exceptional optical properties that significantly outperform conventional fibre structures in key areas, and can guide light either by a modified form of total internal reflection or by exploiting photonic bandgap effects. This has generated enormous interest both within the academic and industrial communities due to novel optical properties that include endlessly single-mode guidance, anomalous dispersion, and mode area tailoring over three orders of magnitude that have many potential applications.
The inclusion of functional materials into holey fibres and other engineered microstructured material to provide specifically tailored metamaterials is of significant technological interest as this allows easy integration into existing optoelectronic systems and devices.
Various techniques addressing this objective have been reported. For example, in an experiment to generate electrically stimulated light induced second harmonic generation in fibres with 50 μm diameter capillaries either side of a germanium-doped silica core, metal electrodes were physically inserted into the capillaries to create the required electrostatic field [1]. A similar technique was used to fabricate a magnetic field guide for atom optics formed by current carrying wires inserted into four parallel holes in the fibre [2]. A cloud of laser cooled 85Rb atoms was coupled to this fibre, and propagated over several centimeters. However, these ‘bespoke’ methods of fabricating composite fibre devices suffer from the obvious drawback that they are not flexible and do not readily lend themselves to large-scale production.
Other work has looked at the properties of polymers incorporated into the voids of microstructured fibres, such as an integrated all-fibre variable attenuator where the refractive index temperature dependence of a polymer introduced into the void regions of a tapered holey fibre is utilised. The polymer was infused into the structure by evacuating one end of the fibre with a pump [3].
A fibre Mach-Zehnder interferometer for electro-optic switching has been reported, in which a low eutectic temperature (137° C.) molten alloy (Bi 43%:Sn 57%) was impregnated under pressure (8 atmospheres) into a twin core optical fibre which also had two capillary channels running parallel to the cores. This pressure was sufficient to infuse 22 m of metal alloy into the fibre capillary (hole sizes ranging from 20 to 40 μm). The internal electrodes were used to apply an electric field preferentially to one of the cores to exploit its weak intrinsic Kerr non-linearity [4].
An alternative application is the inclusion of semiconductor nanomaterials such as CdSe quantum dots and rods into the cylindrical microcavity geometry of a silica capillary fibre, which points towards the potential impact this class of technologically important materials could have on microstructured fibres. The large gain magnitudes of semiconductors and their optical non-linearities have immediate applications in the fabrication of lasers, optical amplifiers, switches etc. The method currently used to impregnate cylindrical microcavities with dyes or quantum dots relies purely on the capillary action of a silica capillary when immersed in a solution of the material, and then allowing the solvent to evaporate inside the fibre [5]. This technique has several shortcomings, not least of which is the length over which material can be infused into the fibre which relies on properties such as viscosity, surface tension and glass wall adhesion characteristics of the solvent.
Related work has been performed to embed CdS quantum dots into the interstitials of a self-assembled synthetic opal by exposing the structure to a vapour of Cd and S for the growth of nanocrystals within the lattice voids [6]. This work has been extended to allow the impregnation of CdSe or Si or Ge within the opal interstitials [7]. However, the results can suffer from only partial and inhomogeneous filling of the opal's interstitial pores. Typically, in order to optimise the optical performance of these “inverted opals”, the underlying silica template is dissolved away, but unless the inverted opal has been properly formed, this results in the collapse of the inhomogeneous semiconductor structure. More importantly, self-assembled structures are often not adequate for photonic devices, since the array of voids cannot be suitably controlled.
Microfabrication of structures inside capillaries has been demonstrated, using liquid flow [8]. The liquid transports reactive species to interfaces in the capillary and relies on microscopic laminar flow which allows liquid streams carrying different reagents to remain separate. The width of the separate streams thus determines the size of features that can be grown, and the resulting structures are limited in the variety and complexity of flow patterns that can form. The method is thus wholly dependent on liquid flow characteristics.
Given the importance of metamaterials and their many potential applications, there is, therefore, a need for an improved technique for their fabrication.